Xr70 carb on a 50


24 replies to this topic
  • mjwwf5

Posted December 17, 2013 - 09:54 AM

#1

As the title says I'm putting an xr70 carb (17mm) on my stock 50. Has anyone does this and know what intake manifold they used? I think I need one that moves the carb to the side like the big bore carbs, but I am not sure where to buy one.

  • Go Big Red

Posted December 17, 2013 - 11:52 AM

#2

As the title says I'm putting an xr70 carb (17mm) on my stock 50. Has anyone does this and know what intake manifold they used? I think I need one that moves the carb to the side like the big bore carbs, but I am not sure where to buy one.

I had the same thing happen when i put an XR70 cylinder and head on my 50.  I am also in the process of trying to find an intake and will let you know what i can come up with.  You may want to ask the folks at DrATV.com



  • mjwwf5

Posted December 17, 2013 - 11:54 AM

#3

I had the same thing happen when i put an XR70 cylinder and head on my 50. I am also in the process of trying to find an intake and will let you know what i can come up with. You may want to ask the folks at DrATV.com

I've seen a full 17mm kit for the xr50 on fast50s.com. I emailed them to see if they would just sell their intake. I'll update when I hear back.

Maybe something like this would work?
http://m.ebay.com/it...44?nav=VI&sbk=1

Edited by mjwwf5, December 17, 2013 - 12:04 PM.


  • Go Big Red

Posted December 17, 2013 - 03:27 PM

#4

Well, my experiment failed miserably.  I mistimed the bike and when I kicked it over, bent a valve.  So, it is back to a 50 again until I can rebuild the 70 head.  Let me know if you hear anything.  The saga continues.



  • Go Big Red

Posted December 17, 2013 - 03:29 PM

#5

I just looked at the adapter and although it may work,it defeats the purpose of the bigger carb.  You want airflow, and just adapting a 70 carb to a 50 intake won't get you what you are looking for.  



  • mjwwf5

Posted December 18, 2013 - 03:26 PM

#6

I got a reply from fast50s and they said they custom make an intake and gaskets for the 50 head. Pretty expensive at $60.

I've read in a couple forums that people used 18mm intakes from bbr or trailbikes. I think they had to drill out the bolt holes to fit though as they won't fit normally.

  • Go Big Red

Posted December 18, 2013 - 07:47 PM

#7

Interesting.  I will give the Brown Brothers a call here soon and ask about their intakes.  I think an intake that tips out the carb would work also, its just a matter of finding one that fits the bolts on the head.



  • dirtbkr188

Posted December 19, 2013 - 05:02 AM

#8

As I recall, the XR/CRF70 carb bolt holes are only 45mm apart, whereas the bigger carbs (18mm Keihin, 19mm Mikuni, and the TrailBikes 20mm ShengWay) are all 48mm apart. Trailbikes makes an intake for the stock 50 head that is match ported to the opening, but those carb mounting bolt holes are also 48mm apart. 



  • mjwwf5

Posted December 19, 2013 - 06:52 AM

#9

Interesting. I will give the Brown Brothers a call here soon and ask about their intakes. I think an intake that tips out the carb would work also, its just a matter of finding one that fits the bolts on the head.

Let me know what you find. I'd like to do this right on mine and not rig it if that's possible.

Also are you porting your head? I've never tackled that before and am wondering it it is necessary.

Edited by mjwwf5, December 19, 2013 - 06:53 AM.


  • hazard88

Posted December 19, 2013 - 02:27 PM

#10

Are you doing this to support a big bore kit?  Just wondering if you are trying to piece two bikes together to get one of them running.  I have a functional stock 50 carb if you need one. 



  • Go Big Red

Posted December 19, 2013 - 04:41 PM

#11

Are you doing this to support a big bore kit?  Just wondering if you are trying to piece two bikes together to get one of them running.  I have a functional stock 50 carb if you need one. 

Nope.  Nothing like that at all.  I have an extra 70 engine with a roasted bottom end, so decided to put the top end on the 50.  Now we are just trying to figure out how to make the 70 stuff fit on the 50 frame.  



  • dirtbkr188

Posted December 20, 2013 - 05:06 AM

#12

An  alternative would be to cut the 70 manifold at the base, rotate the upper part a 1/4" to the right, and reweld it, like I had done to a 50 manifold, shown below on a 70 head. I was curious to see if a stock looking small-block stroker would run with a stock carb and pipe.

 

101_0029-640_zpsa1f58cdf.jpg



  • Go Big Red

Posted December 20, 2013 - 08:46 AM

#13

Although a solution, this is something I was hoping to avoid.  Chicks dig welding skillz, but unfortunately, this is why chicks don't dig me.

 

I think BBR has something that will fit and make this work, but they have about a two ay turn around on emails.  I am in waiting mode.



  • Go Big Red

Posted December 20, 2013 - 11:05 PM

#14

Heard back from Chris Brown and the BBR manifold will not work for the 70 carb.  It is for a larger carb they sell in their kit.  So, to make it work, you would have to fashion something custom like dirtbkr did.  Damn it.  

 

Oh well, need to order a standard 50cc piston and just get the thing running as best we can for the 6 year old.



  • IAoffroader

Posted January 10, 2014 - 03:41 PM

#15

The ATC/TRX 70 intake manifold works perfectly for the Honda 50 engine using the XR70 carb.  I've used this intake on two of my 50's, both 1982 Z50R's. One of the 50's has a Powroll 75cc kit (stock 50 head) and the other has their 80cc kit (XR70 head).  Performance is very good and the power comes on smooth with no bogs.  I used the stock Honda jetting on both carbs.



  • MorganM

Posted January 12, 2014 - 03:59 PM

#16

The ATC/TRX 70 intake manifold works perfectly for the Honda 50 engine using the XR70 carb. I've used this intake on two of my 50's, both 1982 Z50R's. One of the 50's has a Powroll 75cc kit (stock 50 head) and the other has their 80cc kit (XR70 head). Performance is very good and the power comes on smooth with no bogs. I used the stock Honda jetting on both carbs.


Do expect the ATC/TRX 70 intake fits newer 50s as well?

  • IAoffroader

Posted January 12, 2014 - 05:36 PM

#17

I wouldn't know for sure without checking but I don't believe that the intake mounting has changed for the 50 in a long time, if ever.  I imagine that there is a difference in the intake passage diameter between the TRX 70 intake and the Z/XR/CRF 50 but I didn't do any port matching.  I used the TRX 70 intake gasket and heat isolator plate next to the carb.  Good Luck!



  • dirtbkr188

Posted January 13, 2014 - 04:46 AM

#18

The intake mounting is the same, the issue is the carb top and throttle cable hitting the frame. I'm almost positive that the clearance is less on the XR/CRF50 frame than it is on the Z50 frames, and the ATC/TRX manifold stands taller than the 70 one does.



  • IAoffroader

Posted January 24, 2014 - 05:43 PM

#19

You are possibly right about the TRX manifold being taller than the CRF version.  As noted, the reason the CRF 70 manifold doesn't work on the 50  frame is that it positions the carb top so that it interferes with the frame.  The difference between the XR/CRF 70 manifold and the TRX 70 manifold is the TRX version curves out and moves the carb. away from the top rail of the frame - that is why it works.  I'm 95% confident this intake manifold will also work on the later XR/CRF 50 frames. 



  • MorganM

Posted January 24, 2014 - 07:14 PM

#20

Thank you both- super helpful!!





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